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Challenge

1. When we start editing the birthday, the view moves up and the birthday UITextField moves to right above the keyboard;
2. Don't accept inputs when the characters of the UITextField exceed the length of this format: "MM/DD/YYYY";
3. Automatically insert or remove "/" to or from the appropriate positions;
4. Tap on anywhere of the view to end editing.
5. When we end editing the bithday, the view moves down to the original position.